energy loss in heavy ion collisions jana bielcikova (yale university)

Energy Loss in Heavy Ion Collisions

Jana Bielcikova (Yale University)

J. Bielcikova (Yale) 2007 RHIC&AGS Annual Users' Meeting 2

Outline:

• introduction to energy loss• p+p reference data vs pQCD• gluon/light quark energy loss • heavy-flavor energy loss• direct photons

Observables: RAA, particle ratios, two-particle correlations

vs system size, energy, rapidity


• scattering of partons followed by fragmentation jet • can be calculated in perturbative QCD• collinear factorization

High-pT particle production in p+p

c

chbbaa

abcdba

T

hpp

z

Dcdab

td

dQxfQxfdxdxK

pdyd

d

0

/222

)(ˆ

),(),( Parton distribution function Matrix element Fragmentation

function

Measured in DIS e+e-pQCD

a

bc

dhadrons

hadrons

leading particle


Parton energy loss in A+A (“jet quenching”)

A+A collisions: • initial state: similar to p+p

• final state: hard partons traverse medium and lose energy - gluon radiation - elastic collisions with surrounding partons

softening of particle spectra at high pT

• energy loss different for gluon, light and heavy quarks (color factor, dead cone effect)

Goal: Use in-medium energy loss to measure medium properties

c

dhadrons

hadrons

b

a


4 jet quenching schemes:•higher twist expansion Qiu, Sterman, Wang, Wang, Zhang, Majumder,…

•finite temperature field theory Arnold, Moore,Yaffe (AMY)

•opacity expansion: - thin medium/single hard scattering Gyulassy, Levai, Vitev,Djordjevic,… (GLV)

- thick medium/multiple soft scatterings Baier, Dokshitzer, Mueller, Peigne, Schiff (BDMPS)

Armesto, Salgado, Wiedemann (ASW)

• medium properties can be characterized by a single constant:

• static medium: E L2 due to interference effects, expanding medium: E L

2

ˆ qe.g. transport coefficient ‘average kT-kick per mean-free-path’

Radiative energy loss in QCD

energy loss

Salgado, Wiedemann PRD68 (2003) 014008

BDMPS

GLV

Medium-induced radiation spectrum2ˆLqC


p+p reference data


p+p reference @ 200 GeV vs pQCDCompilation by D. d’Enterria nucl-ex/0611012

Good agreement with theory

pT-differential cross section: - jets - charged hadrons - 0 ~0 - direct photons - D,B (non-photonic e-)

- negative charged hadrons =3.2 • 9 orders of magnitude (10mb-10pb) broad range in pT


Proton and pion production in p+p

p+p -> p + X

p+p -> + X

Pion and proton spectra agree with NLO pQCD using the latest AKK fragmentation functions

(Note: p is more sensitive to gluon fragmentation – KKP does not work!)

STAR, PLB 637 (2006)

0

A. Bazilevsky (PHENIX), QM06


• STAR measurement of strange particles in p+p constrained AKK FF

AKK fragmentation functions agree well with both mesons and baryons at mid-rapidity.KKP (Kniehl-Kramer-Potter): NPB 582 (200)

AKK (Albino-Kniehl-Kramer): NPB 734, 50 (2006)

DSV (DeFlorian-Stratmann-Vogelsang): PRD57, 58111 (1998)

Strange particle production in p+pSTAR, PRC 75 (2007)


Hadron production in p+p at y~3BRAHMS, hep-ex/0701041, submitted to PRL

NLO pQCD describes production of pions and kaons well at y~3, but fails to account for large proton yields and small p/p ratio even with AKK FF.


Nuclear modification factorsParticle ratios


RAA in Au+Au @ 200 GeV

Direct photons: • measured p+p reference• RAA() < 1 for pT > 12 GeV/c Isospin effect seen?Ongoing study at √sNN=62 GeV

0 and • Run 5: extended reach in pT out to 20 GeV/c for0 and 15 GeV/c for • both have a common RAA~0.2

T. Isobe (PHENIX), QM’06

ddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

binary collision scaling p+p reference


New! Measured p+p reference at √sNN = 62 GeV instead of previously used ISR data RAA at √sNN=62 GeV is now very close to √sNN=200 GeV !

Energy dependence of RAA


Comparing RAA() to models

minimization fit to obtain the probability of a given parameter values given for probability > 10%

RAA shows only a small sensitivity to model parameters

/fmGeV24q̂6 2

B. Sahlmueller (PHENIX), QM’06

C. Loizides, hep-ph/0608133

2000dy/dN1000 g 1600dy/dN600 g

I. Vitev W. Horowitz


What do we learn from RAA?

~15 GeV

Energy loss distributions very different for BDMPS and GLV formalisms

BUT! RAA is similar

Renk, Eskola, PRC 75, 054910 (2007) Wicks,Horowitz,Djordjevic,Gyulassy

NPA 784, 426 (2007)

thick plasma (‘BDMPS’) thin plasma (‘GLV’)

More differential probes needed!

Gluon jet contribution factor increases from , K, p towards:

e.g. pT = 8 GeV/c: 50% for

90% for p

If

and

At high pT for same beam energy, system and centrality:

RCP() > RCP(p) RCP(K) > RCP()

Gluon vs quark energy loss

4

9~

E

E

q

g

2 ˆ LqCE Rs

AKK = particle + anti-particle AKK (Albino-Kniehl-Kramer): NPB 734, 50 (2006)

Intermediate-pT (pT =2-5 GeV/c): baryon/meson splitting RCP(meson)<RCP(baryon)

High-pT (pT > 5 GeV/c): RCP()~RCP(p) RCP(K)~RCP()

Does it mean similar energy loss of quarks and gluons ?

STAR, PRL 97, 152301 (2006)

RCP of identified particles


Energy dependence: anti-particle/particle ratiosS

TA

R, nucl-ex/0703

040

-/+: independent of pT

p/p: model calculations with/without Eloss do not describe data

X.-N. Wang et al., PRC 70 (2004) – Eloss

Baryon junction and coalescence models describe data at intermediate pT

I. Vitev et al.. NPA 715 (2003) - baryon junction V. Greco et al., PRC 71 (2005) - coalescence

ST

AR

, P

RL

97,

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301

(20

06)

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Rapidity dependence of RAA

y=0 y=1 y=3.1

pions

kaons

protons

RAA independent of rapidity for produced mesons

Au+Au at 200 GeV

I. Bearden (BRAHMS), QM2006


Why is RAA independent of rapidity?

• forward y = a unique information about medium in the longitudinal direction in the early stage

• competing effects: shadowing, multiple scattering, energy loss (GLV), geometry• opacity decreases ~ linearly with y BUT!• shadowing stronger at forward y effects compensate each other RAA independent of y

Barnafoldi, Levai, Papp, Fai, EJP C49 (2007)

L/opacity

pT (GeV/c)


RAA of non-photonic electrons

Note: ~ agreement between STAR and PHENIX

(disagreement is common to p+p/Au+Au and cancels out in RAA)

Models have difficulties to describe

the measured RAA:

• radiative energy loss with typical gluon densities is not enough (I)

M. Djordjevic et al., PLB 632 (2006) 81

• models involving a very opaque medium agree better (II)

N. Armesto et al., PLB 637 (2006) 362

• collisional energy loss/resonant elastic scattering (III,IV,V)

S. Wicks et al., NPA 784, (2007) 426

H. v. Hees, R. Rapp, PRC 73 (2006) 034913

• heavy quark fragmentation and dissociation in medium → strong suppression for c and b (VI)

A. Adil, I. Vitev, PLB 649, (2007) 139


Towards more differential probes …


RAA vs reaction plane

Au+Au collisions at 200GeV

PHENIX, nucl-ex/0611007, submitted to PRC

• factor 2 suppression out-of-plane than in-plane• in plane emission shows no energy loss in peripheral bins.

3<pT<5 GeV/c

In Plane

Out of Plane

L


Path length dependence of energy loss

L = matter thickness calculated in Glauber model

• RAA is universal function of L for all centrality classes and both pT ranges• Little/no energy loss for L< 2 fm Formation time effect? Surface emission zone? v2? Pantuev, hep-ph/0506095,

Shuryak+Zahed, hep-ph/0307267

0-10%

50-60%

PHENIX, nucl-ex/0611007, submitted to PRCAu+Au √sNN = 200GeV


Jet-like correlations at high-pT

• disappearance of away-side correlations observed at intermediate pT

in central Au+Au collisions

• Run 4 statistics: a punch through observed at high pT

- away-side yield is suppressed: RAA ~ IAA

- suppression without angular broadening or medium modification

- RAA ~ IAA in Cu+Cu as well

STAR, Phys. Rev. Lett. 97 (2006) 162301

8 < pT(trig) < 15 GeV/c

increa

sing

pT (ass

oc)

J. Jin, N. Grau, J. Jia, H. Pei (PHENIX), QM2006

Cu+Cu, 200 GeV

4 <pT(trig)<6 GeV/c, 2 GeV/c <pT(assoc)<pT(trig)

STAR, PRL 91, 072304


Away-side di-hadron suppression at high pT

• di-hadrons have a smaller surface bias a “better” differential probe

2-minimum narrower for di-hadrons stronger constraint on density

• extracted medium properties: 0 = 1.68 GeV/fm q = 2.8±0.3 GeV2/fm

Zhang,Owens,Wang,Wang, nucl-th/0701045NLO pQCD + KKP FF + expanding medium

^


Au+Au 0-10%

STAR preliminaryNear-side jet peak

Near-side ‘ridge’

Modified away-side (+ v2)

3 GeV/c < pT trigger

< 4 GeV/c, pT,assoc

. > 2 GeV/c

The near-side jet interacts with the medium!

J. Putschke (STAR), HP2006, QM2006

Di-hadron correlations: near-side peak

What is the ridge?1) Medium heating and parton recombination Chiu & Hwa PRC 72, (034903) 2005

2) Radial flow + high-pT trigger particle Voloshin, nucl-th/0312065 NPA 749, 287 (2005) 3) Parton radiation and its coupling to the longitudinal flow Armesto et al, PRL 93 (2004)

4) Momentum broadening in an anisotropic QGP Romatschke, PRC 75, 014901 (2007)

5) Longitudinal broadening of quenched jets in turbulent color fields Majumder, Mueller, Bass, hep-ph/0611135


Near-side yieldsM. Horner (STAR), QM2006

C. Zhang

• increase of near-side yield at low pT

assoc (zT) observed by STAR and PHENIX

• subtraction of -independent ‘ridge-yield’ recovers centrality-independent jet yield

zT = pT,assoc/pT,trigSTAR preliminary

STAR preliminary

J. Jin (PHENIX), QM2006


Ridge propertiesRidge yield: • increases with centrality• ~ independent of pT

trigger

• pT -spectra are ‘bulk-like’• particle composition in the ridge see talk of M. Daugherity

J. Putschke (STAR), QM’06

Ridge: solid symbolsJet: open symbols

jet yield

jet+ridge

“jet” sloperidge slopeinclusive slope

h-h correlationspT

associated>2GeV/c


Energy content of the ridgeSTAR, Phys. Rev. Lett. 95 (2005) 15230, J. Putschke (STAR), QM2006

0.15 < pt,assoc < 4 GeV/c

4 < pt,trigger < 6 GeV/c

6 < pt,trigger < 10 GeV/c

“Ridge energy”

“Ridge energy”

}

}

• near-side modification in published results also due to ridge• energy content deposited in the ridge is few GeV


The golden channel: -jet

• hard process: pQCD calculations describe well measured data• no surface bias • calibrated probe: ET, = pre-quenched ET,jet

• monochromatic source differential measurement of jet-quenching

has no energy loss in medium

But D(z) is softened in opposing jet

Medium size

X.-N. Wang, Z. Huang, PRC 55, 3047, F. Arleo et al JEHP 0411, 009, T. Renk, PRC 71, 034906

PHENIX

hadrons

b

a


First results on -hadron correlationsJ. Jin, M. Nguyen, N.Grau (PHENIX), QM06 S. Chattopadhyay, F. Benedosso (STAR), QM06

• p+p: yields consistent with expectations from Pythia/HIJING• Au+Au: a hint of away-side modification (?)• ongoing studies in Cu+Cu (STAR/PHENIX)

• statistical and systematic uncertainties are large• We have the tools, we just need more statistics


Summary

THANKS to BRAHMS, PHENIX and STAR for their input to this talk !!!

• high-statistics Run4/Run5 enable exploration of in-medium energy loss in detail:

integrated (RAA) differential (RAA vs reaction plane, correlations)

hadrons identified particles (non-strange, strange, heavy-flavor)

‘the golden channel’ – -jet is emerging

• data trigger lots of interest in theoretical community

• upgrades to PHENIX and STAR will bring us even more exciting results

energy loss in heavy ion collisions jana bielcikova (yale university)

Documents

p pp p p xp p p xpion

p p final state

measured p p reference

medium energy loss

high pt energy loss

particle spectra

fragmentation jet

static medium